Information, and access to it, has received significant attention recently. The building of an "information highway" compared to the national interstate highway system begun in the 1950s has been made a national priority. There are currently three wireline transport elements available for such a highway: (1) fiber optic cable; (2) coaxial cable; and (3) twisted copper pair cable ("twisted pair"). Presently, twisted pair cable predominates, certainly in the local loop portion of telephone networks. Coaxial cable has been used widely by cable television companies and both telephone companies and cable companies have made use of fiber optics for main or trunk line signal transport.
Fiber optic cable can carry more information over a greater distance than coaxial cable, while coaxial cable can carry more information over a greater distance than twisted pairs. Because twisted pair is the predominant local loop technology at least in the telephone industry, attempts have been made to develop technologies which will increase the carrying capacity of copper. In reality, copper wire is a very efficient transport means for traditional telephony services.
Because of their high bit rate transport capability, fiber optic-based systems are preferably to copper-based networks even those using digital compression information services that require true broadband rates require fiber or coaxial cable technology, as a practical matter. Even low-end (i.e., POTS "plain old telephone service") services will reflect a lower per-subscriber cost on fiber, compared to present copper-based delivery systems. Specifically, fiber-based systems that provide residence telephony to groups of 4-8 subscribers with fiber to the curb (FTTC) are expected to achieve cost parity with copper in the near future. However, the cost to replace the existing copper plant in the U.S. with fiber optics is estimated a hundreds of billions of dollars. Thus, the length of time required to achieve this conversion could be decades.
One possible alternative to fiber or copper networks is a hybrid network which utilizes existing facilities and employs fiber optics, coaxial cable and copper wiring. Such a network would allow the delivery of many advanced services and yet be more cost efficient to allow earlier conversion to a broadband network with significant fiber optic capability included. At least one company has announced plans for such a hybrid network (DENVER POST, Apr. 24, 1993, Page C1).
Telephony, in addition to requiring two-way communication, has two other requirements not necessarily addressed by video networks: powering and privacy of communication. In video networks the power to operate the subscriber television set, for example, is provided by the subscriber. That is, the subscriber plugs his or her television and/or video cassette recorder into an electrical outlet which provides power in the subscriber location. In the event of a power outage, for whatever reason, the user is unable to view the television unless he or she has a backup source of power (i.e., battery or generator). Few people have such backup power. In telephony, on the other hand, subscribers expect phone service whether or not electricity is available. The following paragraphs discuss a history of power in the telephony network.
Telephones on the early manual networks had their own battery boxes which contained dry cells. These batteries were used to power the carbon granule microphones. In addition, a hand crank generator in the phone supplied the needed signaling to call others on the same line, or the operator. These two power sources within the telephone allowed a user to originate a call and to talk to other users. Neither of these sources were dependent upon household power, allowing calls to be placed even before rural electrification.
When automatic switching was introduced into the network, the battery box was replaced with a common battery located at the switch, including a common ringing voltage source. The central office switch also needed power to operate and make connections between users. Supplying power to each telephone allowed current flow and the timed interruption of that current (dial pulses) to signal the switch of the user's intentions. In addition, the busy state current could be used by the telephone to power the carbon microphone.
Because of the need to protect the switch and the telephone connections from service interruptions, the power plant at the central office was backed up with large wet cell batteries. These batteries in turn were often backed up with motor-generated sets. Several different voltages are used within the network, but the primary supply is -48 volt direct current (vdc) and .+-.105 volts at 20 Hz.
Over time as the telephone network grew in size and service penetration approached 100 percent, service availability (reliability) became one of the most important obligations of the network. For a time, the telephones in users' homes belonged to the network and were maintained by the network owner. In the past 20 years, the ownership of the telephone has changed again and carbon microphones aren't used anymore. However, the new electronic telephones with their silicon chips still rely on the network to supply power for call supervision and even for memory backup.
Service availability is a responsibility shared by the network and the user. The network is responsible for maintaining the switch and connecting trunks as well as testing and maintaining the individual lines to each user. The user also contributes to service availability by keeping the telephone on-hook when it is not needed, by maintaining premises wiring and terminal equipment in good repair, and by limiting the total quantity of equipment connected to one line.
Maintaining the batteries in the telephone's battery box was difficult. Thus, network power is preferable. First of all, the financial cost associated with placing the terminal power back in the terminal equipment would be huge. The supply and maintenance of the needed batteries would either be forgotten (like those in smoke detectors) or would be eliminated. Both of these results would limit the user's service availability. The second reason that power will likely remain in the network is due to the regulatory bodies who are concerned with "life-line" services. This relates to phone service being perceived as a necessarily as pointed out above. Basic telephone service is expected to be available to everyone at a reasonable cost 24 hours a day.
There are few exceptions. Some services are powered by the user today. As more services are introduced in the future, the user equipment associated with these new services may also be non-network powered. One good example is Integrated Services Digital Network (ISDN) services, whether Basic or Primary Rate Interfaces. With ISDN, the network powers its portion of the circuit and the user powers the terminal equipment. Most data services also fall into this category.
Power can only be provided over a fiber optic network with great difficulty and expense. As discussed above, power can and is easily provided over a copper-based network. There are video systems today which utilize cable phone systems in which telephony is provided over a video network system. However, such systems require power supplied by the subscriber, usually in the form of AC power and (in some cases) batteries at the subscriber premises. In addition, adaptive hardware in the form of converter boxes are needed to utilize the phone system.
Hybrid networks such as that shown in FIG. 1 solve the powering and privacy issues as follows. Power is provided by a separate power cable 20 laid along side the fiber optic and/or coaxial cable. A power source 32 is tapped in the central office 13 and cable 20 provides that power to the optical network unit 15. Thus, power for telephony services is provided from the network as in present networks. Privacy is assured because the remote digital terminal 18 prevents any one subscriber from monitoring any other subscriber.
While suited for its intended purpose, the network architecture of FIG. 1 is costly. If the cost of the network was reduced, then such an architecture would become more feasible.